A CI/SIV integral configuration in which an auxiliary power unit (Static-Inverter, hereinafter referred to as an “SIV” as occasion demands) is connected to an intermediate DC circuit unit (also called as an “intermediate link connection unit”) of a main conversion device (Converter-Inverter, hereinafter referred to as a “CI” as occasion demands) that is a propulsion control device has in recent years become commonly used for a control device for an AC electric vehicle.
In contrast, in the case of a configuration in which the CI and the SIV are configured independently of each other (CI/SIV independent configuration), when a vehicle is stopped, the SIV continues to operate so as to supply power to service equipment (lighting, air-conditioning, etc.) whereas the CI is not required to operate because a main motor (Main Motor: MM) that is a load is stopped and thus both the converter and the inverter are stopped.
In the case of the CI/SIV integral configuration, because the SIV is connected to the output stage of the converter as described above, it is necessary to operate the converter in order to continuously operate the SIV. That is, in the case of the CI/SIV integral configuration, the converter operates even when a vehicle is stopped. Accordingly, it is known that the converter is supplied with electric power for a longer period in the CI/SIV integral configuration than in the CI/SIV independent configuration.
When the converter is supplied with electric power for a longer period, it is thought that this is also disadvantageous in terms of lifetime because electrical stress applied to elements increases.
Here, an example of a method for increasing the lifetime of a main circuit element is disclosed in the following Patent Literature 1, for example. The approach adopted in Patent Literature 1 is to suppress the generation loss of the main circuit semiconductor to prolong the lifetime. When the main circuit semiconductor has, for example, a module structure, it is known that the main circuit semiconductor has a soldered joint portion therein and the soldered joint portion is subjected to a repetitive heat cycle and thus suffers fatigue breakdown. Therefore, it is thought that this method would be an effective way to prolong the lifetime if the influence of the heat cycle can be reduced by suppressing the generation loss.